Xvfb


Xvfb or X virtual framebuffer is a display server implementing the X11 display server protocol. In contrast to other display servers, Xvfb performs all graphical operations in virtual memory without showing any screen output. From the point of view of the X client app, it acts exactly like any other X display server, serving requests and sending events and errors as appropriate. However, no output is shown. This virtual server does not require the computer it is running on to have any kind of graphics adapter, a screen or any input device. Only a network layer is necessary.
Xvfb supports several X Protocol Extensions, such as Compositing and OpenGL GLX support via Mesa.

Usage scenarios

Xvfb is primarily used for testing:
  1. Since it shares code with the real X server, it can be used to test the parts of the code that are not related to the specific hardware.
  2. It can be used to test clients in various conditions that would otherwise require a range of different hardware; for example, it can be used to test whether clients work correctly at depths or screen sizes that are rarely supported by hardware.
  3. Background running of clients.
  4. In General, Running programs that require an X server to be active even when they do not display anything with it.

Usage examples

Screenshot example

As an example, the following sequence of commands runs a virtual framebuffer X server as display :1, runs a program on it, and captures the virtual screen in the file image.xwd using the xwd command:

Xvfb :1 &
xclock -display :1 &
xwd -display :1 -silent -root -out image.xwd

The result can be shown by running the xwud program.
The program xvfb-run is often used to automate the process of finding an available display and managing authentication:

xvfb-run command

Remote control over SSH

Xvfb is also used for remote control. VNC over SSH can be faster than X11 over SSH, specially reducing latency over the internet. In this case, Xvfb is often combined with a lightweight window manager and a VNC server such as X11vnc. A possible sequence of commands to start this on the server is:

export DISPLAY=:1
Xvfb "$DISPLAY" -screen 0 1024x768x24 &
fluxbox &
x11vnc -display "$DISPLAY" -bg -nopw -listen localhost -xkb

The next step is to start a SSH client such as PuTTY with tunneling to localhost port 5900 enabled. A vncviewer can then connect to localhost to get remote control over the server.

ssh -N -T -L 5900:localhost:5900 user@remotehost &
vncviewer -encodings 'copyrect tight zrle hextile' localhost:5900

x11vnc's man page also contains instructions.
Xvnc is very similar to Xvfb.